RFR: 8368740: Serial: Swap eden and survivor spaces position in young generation

[Francesco Andreuzzi]

On Fri, 26 Sep 2025 07:33:19 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Change the relative position of eden and survior spaces in young-gen so that eden can be expanded to fulfill allocation request in heap-near-full scenario. What is on master doesn't allow eden expansion if there is live objs inside from/to after full-gc.
> 
> Test: tier1-8

src/hotspot/share/gc/serial/defNewGeneration.cpp line 345:

> 343: 
> 344:   MemRegion eden_mr{eden()->bottom(), (HeapWord*)_virtual_space.high()};
> 345:   eden()->initialize(eden_mr, eden()->is_empty(), SpaceDecorator::Mangle);

`eden()->is_empty()` should always be `false` after expansion, since `delta_bytes > 0`, right? Could be an assertion

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PR Review Comment: https://git.openjdk.org/jdk/pull/27513#discussion_r2381511734